Cesium-137: A Deadly Hazard

Colin Wessells
March 20, 2012

Introduction

Among the many fission product nuclides, cesium 137
deserves attention because it possesses a unique combination of physical
properties and historical notoriety. It is readily produced in large
quantities during fission, has an intermediate half-life, decays by
high-energy pathways, and is chemically reactive and highly soluble.
These physical properties have made cesium 137 a dangerous legacy of
major nuclear accidents such as Chernobyl, but it has also caused
relatively small incidents as well.

The Dangers of Cesium-137

Cesium-137 is among the most common heavy fission
products. Fission of various isotopes of thorium, uranium, and plutonium
all yield about 6% cesium-137. [1] This high fission yield results in an
abundance of cesium-137 in spent nuclear fuel, as well as in regions
contaminated by fission byproducts after nuclear accidents. [2] The
large quantities of cesium-137 produced during fission events pose a
persistent hazard. Its half-life of about 30 years is long enough that
objects and regions contaminated by cesium-137 remain dangerous to
humans for a generation or more, but it is short enough to ensure that
even relatively small quantities of cesium-137 release dangerous doses
of radiation (its specific radioactivity is 3.2 × 1012
Bq/g). [2-4]

Along with its intermediate half-life, a combination
of high-energy radioactivity and chemical reactivity makes cesium-137 a
particularly dangerous fission product. Cesium-137 undergoes high-energy
beta decay, primarily to an excited nuclear isomer of Barium 137, which
in turn undergoes gamma decay with a half-life of about 150 seconds. [4]
The energies of both the beta decay of cesium-137 and the subsequent
gamma decay of the excited barium 137 are 512 keV and 662 keV,
respectively. [4] In addition, cesium is much more chemically reactive
than many of the transition metal fission products. As a group 1
alkaline metal, elemental cesium is quite electropositive, and is
readily oxidized by water, forming highly soluble Cs+. [5]
For this reason, elemental cesium-137 may contaminate large volumes of
water during nuclear accidents, which are difficult to contain or
process. [6]

Despite its prevalence in spent nuclear fuel and
nuclear waste, cesium-137 is actually extremely rare. Its half-life is
too short for it to persist from natural fission sources, and on earth
it is a synthetic isotope only. Should further nuclear accidents be
avoided, the dangers of cesium-137 will eventually cease.

The Legacy of Cesium-137 After Nuclear Accidents

The Chernobyl nuclear power plant accident and the
less well-known Goiânia scrap metal accident illustrate the true
dangers of cesium-137. During the Chernobyl explosion, about 27 kg of
cesium-137 were expelled into the atmosphere. [2] After the rapid decay
of iodine-131, cesium-137 was the predominant source of radiation in
fallout from the Chernobyl explosion. Particles of the reactor fuel
settled densely within about 100 km or their release, and within a 30 km
radius of the facility, cesium radioactivity due to ground deposition of
fallout particles was over 1.5 × 106 Bq/m2.
[2] Fallout hotspots to the northeast in Belarus, much farther from the
disaster site, were found to have cesium-137 radioactivities of up to 5
× 106 Bq/m2. [2] In comparison, measurements
in southern Sweden, several hundred kilometers northwest (and upwind) of
the disaster found that the ground surface radioactivity due to cesium-137
was only 8 × 104/m2 Bq, and total
radiation doses peaked at only about twice the typical background rate.
[7] In 2002, sixteen years (about one half of a cesium 137 half life)
after the Chernobyl disaster, a 4,000 km2 area still
contained too much cesium-137 to be inhabited or used for agricultural
purposes. [2] Much of this area must remain unpopulated for decades to
come, until several more half-lives of the released cesium-137 have
elapsed.

A much smaller nuclear accident involving cesium-137
occurred in Goiânia, Brazil, in 1987. A thorough account of the
entire affair was published by the International Atomic Energy Agency in
1988. [8] The accident was initiated when two men who were looking for
old equipment to sell for scrap broke in to an abandoned medical clinic.
There, they found a radiation therapy device left behind when the clinic
closed. Upon ripping apart the device, the men discovered about 30 g of
137CsCl. The men were immediately attracted to it because of
its glowing blue color. The owner of a local junkyard purchased the
device from the men, and proceeded to show off the 137CsCl to
friends and neighbors. After several people involved with the looting of
the device and the release of the 137CsCl fell morbidly ill
with radiation sickness, Brazilian authorities declared a local state of
emergency, and within days, the vast majority of the cesium-137 had been
contained. In contrast to the large disaster at Chernobyl, only a few
people were killed or sickened by cesium-137 in Goiânia. Without
the prompt and well-executed response of the Brazilian government, this
incident could have harmed many more people. The Goiânia incident
shows that failure to properly account for seemingly small quantities of
cesium-137 can be deadly.

Treatment of Cesium-137 Ingestion With Prussian Blue

The Goiânia incident also provides insight
about one treatment method for treatment of cesium-137 that achieved
some success. Ferric ferrocyanide, better known as Prussian Blue, is a
metal organic framework material that has large interstices in its
structure. [9] Electrochemists and battery scientists have long
exploited the structure of Prussian Blue in electrochromic devices
because alkaline ions such as potassium and cesium rapidly intercalate
the structure. [10] Prussian Blue has such a strong affinity for these
ions that it was administered as an antidote for cesium-137 exposure
during and after the Goiânia incident. [8] Up to 10 g of Prussian
Blue were administered daily to patients who had suffered large amounts
of cesium-137 exposure. Later analysis of Goiânia survivors found
that the administration of Prussian Blue resulted in a decrease in
cesium-137 exposure by about 70%. [11] Cesium-137 remains an extremely
toxic radioisotope, but Prussian Blue provides some help to those who
have ingested it.

Final Summary

Cesium-137 is an especially dangerous fission product
because of its high yield during fission, moderate half-life,
high-energy decay pathway, and chemical reactivity. Because of these
properties, cesium-137 is a major contributor to the total radiation
released during nuclear accidents. Finally, a discussion of its
practical applications is beyond the scope of this report, but cesium-137
has received some use as a medical radioisotope for cancer therapies
[6,8]. Future research of effective methods for cesium-137 containment
and capture may someday alleviate future nuclear crises.